1. Field of the Invention
The present invention relates to a brushless synchronous motor which has a rotor in a non-circular profile to enable detection of the position of magnetic poles of the rotor, and a driving control apparatus therefor.
2. Description of the Related Art
FIG. 1 is a cross-sectional view illustrating a main portion of a conventionally used four-pole brushless synchronous motor. In FIG. 1, a rotor 1 having a circular profile is embedded with four driving salient magnets 2-5 arranged at angular intervals of 90 degrees such that N-poles and S-poles alternate in the circumferential direction. A stator 6 in turn is provided with six driving/position detection units 10-15, arranged at angular intervals of 60 degrees, each comprising a magneto-electric converting element 7 such as a Hall element, an exciting coil 8, and a driving magnetic pole 9, where outputs of the respective magneto-electric converting elements are connected to associated signal converter circuits (not shown). Each driving magnet pole 9, which is in an inverted C shape, has a magnetic pole portion which protrudes toward the rotor and is would with an exciting coil 8.
With the configuration as described above, each signal converter circuit outputs a detection signal indicative of the polarity of a salient magnetic pole and a rotating angle of the rotor in accordance with a positional relationship between the salient magnet and magneto-electric converting element, associated with the rotation of the rotor 1, such that the rotor 1 is driven to rotate in a predetermined direction by controlling the direction of an exciting current through the exciting coil based on the detection signal.
FIG. 2 shows, for example, detection signals output from the signal converter circuits connected to the magneto-electric converting elements of three adjacent driving/position detection units 10-12, respectively. As can be seen from the figure, the respective detection signals shift in phase by 60 degrees from one another. The polarity and rotating angle of an associated salient magnetic pole is detected from each of such detection signals, and the rotation of the rotor 1 is controlled by controlling the direction of the excitation current through each exciting coil based on the result of the detection.
Hall elements, for example, can be used for the magnetoelectric converting elements shown in FIG. 1, but the Hall elements, which are semiconductor elements, have problems that they present magnetic detection capabilities highly susceptible to ambient temperatures around the elements, and cannot be used under high temperature environments from the relationship with a junction temperature within the semiconductor elements. For this reason, the synchronous motor is limited in its use under high-temperature conditions since the Hall elements are used to detect the position of the rotor, and requires a protection structure against heat generated by the synchronous motor itself and a further special structure for arranging the Hall elements in the stator. Consequently, the motor is complicated in structure due to the above and other various factors, and the complicated structure is deterrent to a reduction in size of the motor.
To overcome the problem mentioned above, Japanese Patent Nos. 3256134 and 2903514 propose methods of detecting the direction of magnetic poles of a rotating rotor by supplying a high frequency detection signal to a separate detection winding provided in a motor winding, and processing a response to the high frequency detection signal. Also, Japanese Patent No. 3254005 proposes a method of detecting the direction of magnetic poles of a rotor by converting excitation power into vectors, and analyzing vector directions of a current and a voltage in an excited state.
However, the former method which involves the detection coil wound together with motor winding detects the influence of the magnetic poles of the rotor to the AC detection signal, and cannot therefore detect the direction of the magnetic poles unless the rotor is rotating at a proper angular velocity, i.e., cannot detect the polarity of the magnetic poles of the rotor when the rotor is at rest.
In the method which relies on the conversion of excitation power into vectors for control, when the rotation of the rotor is controlled at extremely low rotational speeds, an extremely small amount of reverse current is merely generated from each salient magnetic pole of the rotor which is rotating at such low angular velocities, and makes it difficult to discriminate the salient magnetic poles through the vector analysis on the excitation power including reverse power. Accordingly, the rotation of the rotor must be controlled with an open loop at low rotational speeds, but cannot occasionally be controlled with stability at low rotational speeds. Also, the salient magnetic poles present zero angular velocity when the rotor is at rest, so that any counter electromotive force is not generated in the exciting coil, leading to a disabled vector conversion. Thus, the rotor must be rotated at a proper angular velocity irrelevant to the rotation of motor to generate vibrations, and a response to the vibrations must be converted into a vector for determining the polarity of the magnetic poles of the rotor. As appreciated, a rotating angle of the rotor cannot be detected while the rotor is completely at rest.